1. Field of the Invention
The present invention relates to a process for preparing an electrically conductive diamond electrode of diamond particles and a binder. The binder is preferably an organic binder. The present invention also relates to a particulate conductive diamond electrode with a particulate metal deposited on the diamond particles wherein the particles are bound together by a binder. In particular, the present invention relates to particulate platinum (Pt) or ruthenium (Ru) or rhodium (Rh) and noble metal alloys thereof based diamond film electrodes. The diamond electrode can be used in fuel cells, electrosynthesis or electrochemical-based chemical contaminant remediation.
2. Description of Related Art
The present invention uses a deposition process similar to that described by Gruen et al. See for example U.S. Pat. Nos. 5,989,511; 5,849,079 and 5,772,760. The patents to Gruen et al. describe processes for synthesizing relatively smooth polycrystalline diamond films starting with the mixing of carbonaceous vapors, such as methane or acetylene gas, with a gas stream consisting of mostly an inert or noble gas, such as argon, with, if necessary, also small fractional (1-3%) additions of hydrogen gas. This gas is then activated in, for example, in a microwave plasma environment, and under the appropriate conditions of pressure, gas flow, microwave power, substrate temperature and reactor configuration, nanocrystalline diamond films are deposited on a substrate.
Other related patents relating to diamond deposition are U.S. Pat. Nos. 5,209,916 to Gruen; 5,328,676 to Gruen; 5,370,855 to Gruen; 5,462,776 to Gruen; 5,620,512 to Gruen; 5,571,577 to Zhang et al; 5,645,645 to Zhang et al; 5,897,924 to Ulczynski et al and 5,902,640 to Krauss, as well as numerous patents to Asmussen which are all incorporated by reference herein.
U.S. Pat. Nos. 6,106,692 to Kunimatsu et al; and 5,900,127 to Iida et al describe conductive diamond electrodes. G. M. Swain (Wang, J., et al., J. New Mater. Electrochem. Syst. 3 75 (2000) and Wang, J., et al., Electrochem. Solid-State Lett., 3 286 (2000)) describe electrodes with embedded platinum particles produced by magnetron sputtering.
Electrodes consisting of supported metal catalysts are used in a number of industrial processes (e.g., electrosynthesis) and electrochemical energy conversion devices (e.g., fuel cells). The metal catalysts are typically impregnated into the porous structure of several types of sp2 bonded carbon materials; chemically or physically activated carbon, carbon black, and graphitized carbons.1 Activated carbon is the most common type of support, at least in part because of the material's chemical stability in acidic and alkaline environments. The primary role of the support is to finely disperse and stabilize small metallic particles, and thus provide access to a much larger number of catalytically active atoms than in the bulk metal even when the latter is ground into a fine powder (Auer, W., et al., Appl. Catal., A, 173 259 (1998)). Several properties of the support are important; among them porosity, pore size distribution, crush strength, surface chemistry, and microstructural and morphological stability.
The present invention uses electrically conducting diamond thin films (Wang, J., et al., J. New Mater. Electrochem. Syst., 3 75 (2000); Wang, J., et al., Electrochem. Solid-State Lett., 3, 286 (2000); and Witek, M., et al., J. Wide Bandgap Mater. Vol. 8 No. 3-4 171-188 (Jan/Apr 2001)). The use of electrically conducting microcrystalline and nanocrystalline diamond electrodes in electrochemistry is a relatively new field of research (Xu, J., et al., Anal. Chem. 69, 591A (1997); Swain, G. M., et al., MRS Bull., 23, 56 (1998); Tenne, R., et al., Isr. J. Chem. 38 57 (1998); Pleskov, Y. V., Russian Chemical Reviews 68 381 (1999); Vinokur, N., et al., J. Electrochem. Soc. 143 L238 (1996); and Rao, T. N., et al., Anal. Chem. 71 2506 (1999)). The properties of this new electrode material make it ideally suited for electrochemical applications, particularly demanding ones (i.e., complex matrix, high current density, and potential, high temperature, extremes in pH, and the like). Recent work has shown that nanometer-sized dispersions of Pt can be incorporated and anchored into the surface microstructure of boron-doped microcrystalline diamond thin film electrodes (Wang, J., et al., J. New Mater. Electrochem. Syst., 3, 75 (2000); Wang, J., et al., Electrochem. Solid-State Lett., 3 286 (2000); and Witek, M., et al., J. Wide Bandgap Mater. Vol. 8 No. 3-4 171-188 (Jan/Apr 2001)). The diamond film serves as a host for the catalyst particles providing electrical conductivity (est. ,0.1 Ω cm), thermal conductivity, and dimensional stability. The microstructure and morphology of the diamond, as well as the electrocatalytic activity of the Pt particles, were observed to be very stable during extended electrolysis as no degradation of either was detected after 2000 potential cycles between the hydrogen and oxygen evolution regimes in 0.1 M HClO4 at room temperature (1-6 mA/cm2) (Wang, J., et al., Electrochem. Solid-State Lett., 3 286 (2000)). Importantly, the metal catalyst exposed at the surface is in electronic communication with the current collecting substrate through the boron-doped diamond film, and is electroactive for the underpotential deposition of hydrogen (Wang, J., et al., J. New Mater. Electrochem. Syst., 3, 75 (2000); Wang, J., et al., Electrochem. Solid-State Lett., 3 286 (2000); and Witek, M., et al., J. Wide Bandgap Mater. Vol. 8 No. 3-4 171-188 (Jan/Apr 2001)), the reduction of oxygen, and the oxidation of methanol (Wang, J., et al., J. New Mater. Electrochem. Syst., 3 75 (2000); and Wang, J., et al., Electrochem. Solid-State Lett., 3 286 (2000))
Other art related to forming and/or doping of diamond is described in U.S. Pat. Nos. 2005/0016445 A1 and 2003/0200914 A1 to Noguchi et al; 2001/0001385 A1 to Nakamura et al; 2005/0042161 A1 to Carlisle et al; 2005/0109265 A1 to Linares et al, all of which are incorporated by reference herein.
Polymer electrolyte membrane fuel cells (PEMFCS) are a promising energy technology, particularly for transportation applications (Roen, L. M., et al., Electrochem. Solid-State Lett. 7, A19 (2004)). Membrane electrode assemblies are used in such fuel cells. Electrocatalytic metal particles supported on carbon are used as the cathode and anode in MEAs. The long-term stability of the MEAs is a concern due to the degradation of components in either oxidizing or reducing environments accompanied with electrode potentials induced by the cell reaction (Roen, L. M., et al., Electrochem. Solid-State Lett. 7, A19 (2004); Paik, C. H., et al., Electrochem. Solid-state Lett. 7, A82 (2004); and Kangasniemi, K. H., et al., J. Electrochem. Soc., 151 E125 (2004)). Typical sp2-bonded carbon support materials, such as activated carbon, carbon black, and graphitized carbons, have high electrical conductivity and surface area but are susceptible to microstructural and morphological degradation under oxidizing conditions (Auer, E., et al., Appl. Catal. A, 173, 259 (1998); and Steele, B.C.H., et al., Nature (London), 414, 345 (2001)). For example, degradation of the cathode in fuel cells occurs via oxidation reactions that alter the surface chemistry and microstructure. Ultimately, the oxidation can cause corrosion via carbon gasification reactions. Degradation of the electrocatalyst support is a problem because it leads to loss of electrocatalyst activity, increased ohmic resistance, and reduced operational efficiency of PEMFCs (Roen, L. M., et al., Electrochem. Solid-State Lett. 7, A19 (2004); Paik, C. H., et al., Electrochem. Solid-state Lett. 7, A82 (2004); and Kangasniemi, K. H., et al., J. Electrochem. Soc., 151 E125 (2004); Antolini, E., J. Mater. Sci., 38, 2995 (2003); and Appleby, A. J., Corrosion (Houston), 43, 398 (1987)).
Given the corrosion susceptibility of conventional carbon support materials, there is a technological need for advanced support materials that are morphologically and microstructurally stable during exposure to aggressive chemical and electrochemical environments.